1. Field of the Invention
The present invention relates to an alternating-current motor control apparatus.
2. Description of the Related Art
Maximum torque control is known as a method for efficiently driving a motor having magnetic saliency. In maximum torque control, a maximum torque is generated for a constant output current. An output current command is divided into current command components for a d-axis current (current that influences magnetic flux) and a q-axis current (current that influences a torque), and current control is performed on the basis of the current command components that are divided from each other. When Is is a current amplitude, Ld is a d-axis inductance, Lq is a q-axis inductance, and Φa is an induced voltage constant, an optimum current phase angle β (phase angle between a q-axis and a current vector) that satisfies the maximum torque condition is calculated as follows:
                    β        =                              sin                          -              1                                (                                                    -                                  Φ                  a                                            +                                                                    Φ                    a                    2                                    +                                      8                    ⁢                                                                  (                                                                              L                            q                                                    -                                                      L                            d                                                                          )                                            2                                        ⁢                                          I                      s                      2                                                                                                          4              ⁢                              (                                                      L                    q                                    -                                      L                    d                                                  )                            ⁢                              I                s                                              )                                              (          1          )                ⁢                                      
A d-axis current id is determined as in Expression (3) using Expression (2), which shows the relationship between currents.
                                          I            s                    =                                                    i                d                2                            +                              i                q                2                                                    ,                              i            d                    =                                    -                              I                s                                      ⁢            sin            ⁢                                                  ⁢            β                          ,                              i            q                    =                                    I              s                        ⁢            cos            ⁢                                                  ⁢            β                                              (        2        )                                          i          d                =                                            Φ              a                                      2              ⁢                              (                                                      L                    q                                    -                                      L                    d                                                  )                                              -                                                                      Φ                  a                  2                                                  4                  ⁢                                                            (                                                                        L                          q                                                -                                                  L                          d                                                                    )                                        2                                                              +                              i                q                2                                                                        (        3        )            
Japanese Unexamined Patent Application Publication No. 2003-259680, for example, proposes the following method. That is, in the case where Is is provided as a current command value for a motor, the amplitude Is of the current command is divided into a d-axis current id and a q-axis current iq on the basis of the phase angle β and the relationship shown in Expression (2). Then, current control is performed for each of the d-axis current and the q-axis current. In the case where iq is provided as a q-axis current command value, iq is substituted into Expression (3) to obtain id as a d-axis current command value. Then, current control is performed for each of the d-axis current and the q-axis current.
The current phase angle at which the maximum torque can be obtained and the relationship between the d-axis current id and the q-axis current iq are determined in the form of a data table through direct calculations or from motor parameters, and are used to calculate the d-axis current command value and the q-axis current command value for obtaining the maximum torque.
In addition, Japanese Unexamined Patent Application Publication No. 2000-209886, for example, proposes a method for causing a conduction phase to approach a current phase angle corresponding to a minimum current by changing the conduction phase by a constant amount of change at a predetermined time period using voltage conduction phase information and current information with respect to the rotor position.